Nonie, whose real name is Norah, is a 13-year-old neurodivergent girl living in a settlement on the roof of the American Museum of Natural History. She possesses a physical attunement to weather and water quality, allowing her to predict severe storms and detect contamination. She meticulously documents her observations in a volume she calls the Water Logbook, carrying on the preservation habits she learned from her late mother. Her encyclopedic memory helps her map the flooded world around her.

Bix, whose given name is Beatrice, is Nonie's 16-year-old sister. Because she remembers the world before the collapse more clearly than Nonie does, she carries the trauma of a childhood near-drowning that left her with a paralyzing phobia of water. When not near water, she demonstrates a brutal willingness to confront physical danger and investigate dark, unfamiliar spaces. She took her nickname from an old jazz record belonging to her mother.

Keller is a former entomologist who serves as a nurturing and stabilizing presence for Nonie's family. As a Black man surviving in a ruined society where white supremacy has resurged, he approaches unfamiliar survivor settlements with extreme caution. He understands Nonie's neurodivergence well, having invented a memory game called Animal in Mind to help her process information and connect with others.

Father, whose given name is Alan, is an expert in Indigenous American architecture who built the longhouse shelter at the Amen settlement. He operates with a brash confidence that establishes him as a leader, though his impulsiveness occasionally endangers the group. He frequently looks backward to the pre-collapse world with heavy nostalgia, sometimes prioritizing sentimentality over immediate safety.

Mother, named Deirdre, is the late climate scientist who helped found the Amen settlement before passing away from a terminal kidney disease. She possessed the original keys to the museum and established its central logbook system to preserve artifacts for the future. She recognized Nonie's neurodivergence early, nurturing her scientific curiosity and telling her stories of a research vessel called the Sally Ride.

Childs is the leader of the Hancock settlement, projecting a soft and polite affect that masks a deeply controlling nature. He operates an oppressive, debt-based servitude system, requiring anyone who receives medical care or shelter to work indefinitely to repay him. He views survivors purely in terms of the labor they can provide his community.

Mano is Angel's nephew who fled Puerto Rico just before the collapse to join the Amen settlement. He forms a close childhood bond with Bix and shows exceptional patience toward Nonie's communication differences. His tragic death during a hunting excursion leaves a profound gap in the community and permanently alters the dynamics between the survivors.

Angel is Keller's late partner who helped establish the pharmacy and classroom at the museum settlement. She suffered a pregnancy loss during her dangerous journey to the museum and later succumbed to a mosquito-borne illness. Her passing threw Keller into a deep depression and removed a vital source of care from the community.

Mary is a medic of Indigenous descent who lives at the heavily guarded Feral House. She takes her Hippocratic oath seriously, refusing to carry a weapon despite the violent world outside. She frequently clashes with her settlement's leader over the distribution of medical supplies but remains essential to the group due to her specialized skills.

Esther is the primary doctor at the Hancock settlement. Though she helped build the community alongside its current leader, she recognizes the exploitative nature of its operations and quietly works to protect vulnerable residents. She maintains a covert connection to the outside world, holding information about a long-lost research vessel.

Poppy is an archery teacher living at the military-controlled Feral House. Having lost her own children to an illness, she carries a residual maternal grief that prompts her to bend her community's strict isolationist rules and offer temporary shelter to young travelers.

Jared is the militaristic leader of the Feral House, an outpost occupied primarily by former US Army members. He is fiercely isolationist and operates on white supremacist beliefs, hoarding resources and violently repelling outsiders from his fortified property.

Byron is a young resident of the Hancock settlement who assists the medical staff. He was brought to the community as an infant and works quietly under the radar of the settlement's exploitative leadership.

Darling is a resident of Hancock who communicates exclusively by singing snippets of songs. He arrived at the settlement years ago carrying an infant and actively hides from the community's oppressive leadership, living on the margins of the compound.

Virginia is a crew member aboard the research vessel Sally Ride. She travels periodically to mainland settlements to make contact with survivors and recruit individuals who show potential for scientific observation and documentation.

Jess is an Amen resident who grew up in Alaska. She brings tracking and medical experience to the community, helping organize the settlement's medicinal supplies and teaching practical navigation skills to the younger members.

Beaumont is a resident of the Amen settlement who lives on the museum roof with his partner, Jess, and their young baby.

Evangeline is the infant child of Jess and Beaumont, living within the fragile safety of the Amen settlement.

Sergio is a member of the Amen community who contributes to the settlement's survival by teaching other residents how to make functional arrowheads from knapped flint.

Douglas is a resident of the Cloisters who acts as a greeter for newcomers. He provides tours of the facility, including the electrically lit scriptorium where the community collects and records the stories of passing travelers.